Non-dispersive infrared (NDIR) sensors are a low-cost way to measure carbon dioxide levels in ambient air. However, low-cost NDIR sensors have historically been discounted by scientists who consider laser cavity-ring down spectroscopy (CRDS) analyzers the gold standard for CO2 measurement.
That’s why we were interested in reading a paper titled “Performance and Environmental Correction of a Low-Cost NDIR CO2 Sensor” by Cory Martin. The paper was published as part of his Master’s Thesis for the Department of Atmospheric and Oceanic Science at the University of Maryland.
Mr. Martin compared the accuracy of SenseAir’s K30 CO2 sensor to a laser CRDS CO2 analyzer. After working with both products Mr. Martin concluded that:
“Once correcting for a zero-offset, and performing a regression analysis, the practical accuracy of these [K-30] sensors is less than five parts per million…With errors in this range, these instruments could be used in a variety of scientific applications, including one where the cost allows for the spatial density of observations to better represent an area’s CO2 concentration than that of a single high-accuracy observation site.”
Martin further concluded that the K30 sensor “has the potential to compliment traditional higher accuracy but high cost instruments in certain CO2 monitoring applications.”
NDIR and CRDS Technology Compared
Both NDIR and CRDS depend on carbon dioxide’s absorption of light at the 4.26 µm wavelength. A light source is emitted at one end of a chamber and measured at the other. According to the Beer-Lambert law, the absorption can be used to compute the number of CO2 molecules in a sample.
Where the technologies differ is that the CRDS uses time-based measurement to determine the amount of CO2 in the sample whereas NDIR uses the intensity of light. In addition, CRDS uses a longer path (the cavity ring) which theoretically results in higher sensitivity. In some cases, CRDS can be used to measure CO2 in the parts-per-billion (ppb) range.
While past studies have shown that in laboratory conditions NDIR sensors can approach CRDS sensors in terms of accuracy, the SenseAir K30 shows that it can also do it at a price point available at any budget.